Guards

gistfile1.txt

data MappingRow = MappingRow { date :: Date, prod :: Sym, tenor1v :: Sym, tenor2v :: Sym } deriving Show

convertRow :: [MySQLValue] -> IO MappingRow
convertRow sqlRow = eitherStrToMonadError $
  -- I have to define some otherwise-useless x just so that I can use guards. Is there a better way?
  -- Why is the syntax for guards so restrictive?
  let x
        | [MySQLInt32U date, MySQLText prod, MySQLInt16U tenor1v, MySQLInt16U tenor2v] <- sqlRow = do
            prod' <- maybeToEither "Bad symbol" $ symFromStrMay prod
            let (tenor1v', tenor2v') =
                  (tenor1v, tenor2v) & each %~ (prod' <>) . symFromStrEx . sformat (left 4 '0')
            pure $ MappingRow (yyyymmdd . fromIntegral $ date) prod' tenor1v' tenor2v'
        | otherwise = fail "Bad column count or column types"
  in x